2. Field of the Invention
The present invention relates to a developer composition and more specifically to a toner powder for the development of latent electrostatic or magnetic images, and to a process for fixing toner developed images on an image receiving material.
2. Discussion of the Related Art
There is disclosed in British Patent 1,373,220 a toner powder comprising a polyester resin based on an etherified diphenol and a dicarboxylic acid, and, if required with acids having more than two carboxyl groups or alcohols having three or more hydroxyl groups. It has been found in practice that in the preparation of such thermoplastic resins it is difficult to control the spread in thermal properties of the resulting product. Another disadvantage of the use of such thermoplastic resins in the preparation of toners, is that despite the fact that the fixing temperature of such a toner powder prepared with these resins can be reduced, for example, by controlling the molecular weight distribution thereof, the storage stability, is adversely affected by such adaption.
Furthermore, in order to function properly, toner powders must satisfy a large number of other requirements. The conventional method of preparing a toner powder is to mix the constituents in the melt, cool the melt, and then grind and screen it to the correct particle size. The toner must, accordingly, be well adapted to grinding and satisfy certain requirements with respect to toughness and brittleness. During storage the toner powder must also be stable over a wide temperature range and at extreme relative humidities in order to avoid caking. In addition, the toner powder must not agglomerate under conditions prevailing in a developing device, such as mixing, high temperature, and so forth. It has been found in practice that a glass transition temperature higher than 40.degree. C. is favorable to the avoidance of agglomeration. Furthermore, deposition of toner resin on a photoconductor affects the photoelectric properties of the photoconductor. Accordingly, the toner powder should not leave any non-removable residues on the photoconductor.
The toner image should also be capable of being satisfactorily fixed on a receiving material. The toner image should be fixed so that it is scarcely removed, if at all, under mechanical loads such as by folding and rubbing. The fixing temperature in these conditions should be as low as possible so as to satisfy minimum energy consumption.
The working range of a toner powder should, preferably, be so wide that any temperature inequalities occurring in the fixing station are taken into consideration. The working range of a toner powder is defined as the temperature range between the lower fusing limit, the lowest possible fixing temperature at which the toner image is still adequately fixed, and the upper fusing limit, the maximum fixing temperature at which, using for example the hot-roll fixing method, no toner is deposited on the fixing roller (the "hot roll").
It must also be possible to provide both sides of a receiving material with a toner image. With double sided or "duplex" copying, it is a conventional practice to provide first one side of the receiving material with a toner image, to fix this toner image on the receiving material, to turn the receiving material over, and then provide the other side with a toner image and fix that toner image. This may result in the deposition of toner powder from the toner image on the pressure rollers, which would necessitate regular cleaning of such rollers, and may also cause soiling of subsequent copies. During the last fixing step the first toner image fixed must not be subjected to deformation or become detached from the paper. This means that the softening range and the adhesive and cohesive properties of the toner powder must satisfy certain requirements. Moreover, after the heat treatment, the fixed toner image must rapidly become permanent and lose its tackiness in order to avoid any damage to the toner image on transport through the copying machine.